The present invention is generally directed to a balanced tone application system and in particular to a balanced tone application system for use in a telephone exchange for applying signal tones to transmission line pairs in a balanced manner.
In order to provide service to telephone subscribers it is necessary to apply signal tones to the transmission line pairs coupled to the subscriber receivers. Such tones include dial tones, busy tones, or the like.
Because transmission line pairs are usually grouped immediately adjacent to one another to form rather large cables, when tones, which are AC voltages, are applied to transmission line pairs in an unbalanced manner, crosstalk to other adjacent transmission line pairs occurs. Applying tones in an unbalanced manner requires the tone to be coupled to one side of the line pair while it is returned to ground on the other side of the line pair. The crosstalk which occurs between adjacent transmission line pairs when tones are applied in an unbalanced manner has a serious effect on telephone communication.
To overcome the problems which ensue in supplying tones to transmission line pairs in an unbalanced manner, tones have been applied in a balanced manner wherein an equal amount of tone power is applied to both sides of the line. This avoids crosstalk to adjacent pairs in a cable.
In the past tones have been coupled to telephone lines in a balanced manner by using a floating tone generator which is transformer coupled to the transmission line pairs. Because a single tone generator is utilized by many transmission line pairs, isolation must be provided within each line of the transmission line pairs. The necessary isolation has been provided by using a pair of resistors, a pair of capacitors, and a pair to make contacts. While the prior art methods have been technically satisfactory, they have been expensive. The required capacitors are in themselves quite expensive and also occupy a disproportionate amount of space on the printed wiring boards as compared to the printed board space occupied by other components. Thus, not only is the prior art method expensive but it is also cumbersome to use in commercial applications.
It is therefore a general object of the present invention to provide a new and improved balanced tone application system for a telephone exchange.
It is a more particular object of the present invention to provide a balanced tone application system for use in a telephone exchange which does not require expensive and bulky capacitors and resistors for isolation.